[en] We test the method of infinitesimal unitary transformations recently introduced by Wegner on the Anderson single impurity model. It is demonstrated that infinitesimal unitary transformations in contrast to the Schrieffer endash Wolff transformation allow the construction of an effective Kondo Hamiltonian consistent with the established results in this well understood model. The main reason for this is the intrinsic energy scale separation of Wegner close-quote s approach with respect to arbitrary energy differences coupled by matrix elements. This allows the construction of an effective Hamiltonian without facing a vanishing energy denominator problem. Similar energy denominator problems are troublesome in many models. Infinitesimal unitary transformations have the potential to provide a general framework for the systematic derivation of effective Hamiltonians without such problems. Copyright copyright 1996 Academic Press, Inc

[en] The paramagnetic phase of the two-channel Anderson lattice model in the Kondo limit is investigated in infinite spatial dimensions using the noncrossing approximation. The resistivity exhibits a Kondo upturn with decreasing T, followed by a slow decrease to a finite value at T=0. The decrease reflects lattice coherence effects in concert with particle-hole symmetry breaking. The magnetoresistance obeys an approximate scaling relation, decreasing towards coherent Fermi liquid behavior with increasing field. The magnetic field induces a Drude peak in the optical conductivity. copyright 1997 The American Physical Society

[en] It is argued on the basis of the Chirikov overlap of resonances criterion for the development of chaos and ergodicity that Anderson localization of the phonon modes or the spin-wave modes (in the case of ferromagnetic resonance) can result in a large reduction in the rate of energy absorption by a solid from an oscillating external field. This phenomenon is illustrated by numerical calculations on simple model systems. (c) 2000 The American Physical Society

[en] We report on a study of charge transport through superconductor-insulator-superconductor and normal metal endash insulator endash superconductor structures (SIS and NIS junctions, respectively) where the insulator is of the Anderson type. Devices which are characterized by a junction resistance larger than 10 kΩ show behavior which is typical of Giaever tunnel junctions. In structures having smaller resistance, several peculiar features are observed. In the SIS junctions, Josephson coupling is detected over distances much larger then the typical insulator localization length. In addition, a series of resistance peaks appears at voltages of 2Δ/n, where Δ is the superconducting gap. The NIS Junctions exhibit a large resistance dip at subgap bias. We discuss possible interpretations of these findings and suggest that they may result from the presence of high transmission channels through the barrier region. copyright 1997 The American Physical Society

[en] The Kondo properties of the impurity Anderson model provide a theoretical framework for relating thermodynamic and angle integrated electron spectroscopy data in many heavy fermion materials. We describe the success and the challenges of this approach, summarize a detailed analysis of CeSi₂, and give a perspective on the relation to the lattice Anderson model. (c) 2000 American Institute of Physics

[en] The out-of-equilibrium transport properties of a double quantum dot system in the Kondo regime are studied theoretically by means of a two-impurity Anderson Hamiltonian with interimpurity hopping. The Hamiltonian is solved by means of a nonequilibrium generalization of the slave-boson mean-field theory. It is demonstrated that measurements of the differential conductance dI/dV , for appropriate values of voltages and tunneling couplings, can give a direct observation of the coherent superposition between the many-body Kondo states of each dot. For large voltages and arbitrarily large interdot tunneling, there is a critical voltage above which the physical behavior of the system again resembles that of two decoupled quantum dots. (c) 2000 The American Physical Society

[en] Continuous one-dimensional models supporting extended states are studied. These delocalized states occur at well-defined values of the energy and are consequences of simple statistical correlation rules. We explicitly study alloys of δ-barrier potentials as well as alloys and liquids of quantum wells. The divergence of the localization length is studied and a critical exponent (2)/(3) is found for the δ-barrier case, whereas for the quantum wells we find an exponent of 2 or (2)/(3) depending on the well's parameters. These results support the idea that correlations between random scattering sequences break Anderson localization. We further calculate the conductance of disordered superlattices. At the peak transmission the relative fluctuations of the transmission coefficient are vanishing. copyright 1997 The American Physical Society

[en] The bilayer manganite La_1.2Sr_1.8Mn₂O₇ exhibits a transition between nearly insulating paramagnetic and metallic ferromagnetic states at T_C=126K. We use magnetic neutron scattering to show that the paramagnetic state contains long-lived antiferromagnetic clusters coexisting with ferromagnetic critical fluctuations. The principal implication is that at least in two dimensions, Anderson localization effects must also be taken into account in attempts to explain giant magnetoresistance in manganites. copyright 1997 The American Physical Society

[en] We study the transition from a dense continuum to a sparse quasicontinuum in the Fano problem. Transmission measurements on epitaxial layers of GaAs in a high magnetic field and calculations of the optical absorption show how the Fano interference disappears as quantum confinement discretizes the continuum states. The transition between quasi-one-dimensional and quasi-zero-dimensional systems occurs at length scales which are unusually large for optical experiments. copyright 1997 The American Physical Society

[en] We studied by computer simulation the effects of Coulomb interactions on the properties of strongly localized Anderson insulators. We took full account of many-body effects by considering the many-electron configurations of the system rather than single-particle states. We developed an algorithm to obtain the configurations and energies of the low-lying system states, and from there the conductivity. At low-temperatures T, we found that the conductivity was proportional to exp(-T₀/T)^1/2. Many-electron transitions were seen to be important at very low temperatures. In this regime, T₀∼0.61, which is much smaller than predicted by one-electron theory. Experimental results which use the predicted T₀ to obtain localization radii must therefore be reinterpreted. copyright 1997 The American Physical Society